Seasonal energy storage

ASHRAE Journal, Jan, 2009 by Kurt Roth, James Brodrick

In addition, cold storage pits exist, including a snow storage pit used to cool a hospital in Sweden. During winter, natural and man-made snow fills the pit to create a frozen reservoir that is covered with wood chips to insulate the reservoir. During the cooling season, pumps extract the melt water from the pit and use it to cool the hospital. (3,6)

Many systems also use a diurnal storage component (typically a water tank) to complement the STES. This second, smaller tank acts as a thermal buffer between the STES and the heat sources and sinks, i.e., it can accept heat from the thermal resource--and transfer heat to the heating loads--at higher rates than a BTES can achieve. For example, the STES often cannot accept the peak output of solar thermal collectors during the summer. Instead, the hot water generated by the collectors primarily flows to a sizeable water storage tank during the middle of the day while the STES charges at a slower rate from the diurnal storage and the collectors throughout the day. (8,9)

Other concepts considered and, to varying degrees deployed include caverns, in-soil ducts, above-ground water tanks, rock storage with air circulation, latent heat storage (using phase change materials) and thermochemical heat. (3,4,6)

STES systems also include backup thermal energy sources, such as gas boilers and district heating systems.

STES systems may or may not have a heat pump to augment the quality of the thermal energy harvested. Each approach has its pros and cons. On one hand, a heat pump enables use of lower quality (e.g., for heating, lower storage temperature) resources, increasing the storage capacity ([dagger]) and decreasing the first cost of the storage itself. On the other hand, it incurs the first cost of the heat pump, which can be significant. Designing a system without the need for a heat pump avoids its first cost,1 but also requires that the systems have stored energy of sufficient quantity and quality throughout demand periods to avoid significant use of backup heating or cooling sources. This usually increases the storage temperature and/or size of the STES; the former increases thermal losses, while the latter increases first cost.

Although in theory STES can be applied at any scale ranging from a single home to a sizeable community, (10) its economics--and its efficiency--improve appreciably with scale. (5,7) As a result, most systems are built for applications with higher levels of heating demand, typically several hundred to thousands of kW of maximum thermal output. (5) Consequently, the rest of this article focuses on larger-scale STES.

[FIGURE 1 OMITTED]

Energy Savings Potential

STES saves energy by storing thermal energy that would otherwise be wasted and using it to meet a significant portion of building space heating, water heating, and/or cooling loads. As such, the energy savings potential of STES largely depends on the portion of these loads that the stored energy can supplant. In turn, this depends on the capacity of the STES relative to the loads and the efficiency of the STES, both of which are part of the economic optimization of a specific application.